A sealed diaphragm pressure sensor has been proposed in JP-A-7-243926 (Patent Document 1). According to this pressure sensor, a pressure detecting chamber is sectionally formed by a metal diaphragm for receiving measured pressure in a case, a semiconductor chip having a pressure-sensitive portion is equipped in the pressure detecting chamber, and electrically insulating pressure transmitting liquid for transmitting the pressure received by the metal diaphragm to the semiconductor chip is sealingly filled in the pressure detecting chamber.
The pressure sensor includes a semiconductor chip having a pressure-sensitive portion, such as a diaphragm or the like formed on a silicon substrate. The pressure sensor also includes an integrated sensor chip for signal processing comprised of an electrical circuit disposed around the pressure-sensitive portion at a surface site of the semiconductor chip. The electrical circuit comprises transistor elements, etc. which are formed at the peripheral portion of the semiconductor chip by the semiconductor manufacturing technique. Furthermore, the surface of the electrical circuit is covered by protection film formed of silicon nitride film or the like in the semiconductor chip.
In the pressure sensor as described above, the measured pressure is applied to the surface of the metal diaphragm at the opposite side to the pressure detecting chamber, and the measured pressure is applied from the metal diaphragm through the pressure transmitting liquid to the pressure-sensitive portion of the semiconductor chip.
An electrical signal output from the pressure-sensitive portion on the basis of the measured pressure is amplified and adjusted by the electrical circuit and externally transmitted.
However, in the pressure sensor described above, the pressure transmitting liquid in the pressure sensor may be polarized under an electrostatic environment, so that the surface site of the semiconductor chip is electrified. Further, as disclosed in Patent Document 1, oil or the like is frequently used as the pressure transmitting liquid. Accordingly, charges occurring due to the electrification invade circuit elements such as a transistor element or the like constituting the electrical circuit, and cause malfunction of the electrical circuit of the semiconductor chip, thereby inducing variation of the sensor characteristics.